For testing and operating of mechanical systems, in particular for rotor vibration measurement, there is a demand for low-cost and minimally intrusive instrumentation.
Traditionally, phonic wheels, gears/gear rims, light and dark stripes or other specifically designed periodic structures are placed on rotors where the vibration measurement has to be made. The passage of each individual structure is detected by a sensor. The rotor moves with respect to the sensor. Limitations on the accuracy of the stripe size directly translate in a severe limitation to measurement accuracy. In fact, only in theory are stripes or blocks of a rotor marking or of a gear rim uniformly spaced.
Rotor vibration measurements are mainly related to torsional vibrations (vibrations around the axis of rotation), however rotor vibration may easily exhibit torsional-lateral coupling. Therefore, the general term “rotor vibration” is used here. Typical measurement campaigns on rotating machinery include an angular reference for phase synchronization between vibrations and angular position. Such synchronization does not pose particular difficulties, however it also brings limited insights on the rotor performance. Detailed vibration measurements, such as rotor instantaneous angular speed, twist angle, torque, stiffness and motion transmission error, require, however, high measurement accuracy. With better accuracy, the evaluations on performance attributes (e.g. consider early fault detection to evaluate machine health)are finer and more sensitive.
For example, to achieve the required accuracy, optical encoders or laser vibrometers are needed. An encoder requires an accessible shaft or substantial space to be mounted on a rotor; both of them are typically not available since packaging space is kept to a minimum. Furthermore, encoders are not available with custom diameter sizes and a redesign is typically required to make mounting possible. Such obstacles often cannot be overcome; therefore, valuable detailed measurements cannot be performed using encoders. Laser vibrometers can perform a contactless measurement and bring an improvement on ease of instrumentation. However, this comes at the price of a reduced accuracy and a substantially increased cost (especially important when multiple locations need to be measured simultaneously). Furthermore, the scanning head of a laser vibrometer has a considerable size. Moreover, a few requirements for beam positioning introduce significant constraints on reachable locations. Again such limitations often do not allow performing the detailed measurements.
A further technique can be mentioned for such measurement. It uses paired accelerometers tangentially mounted on the circumference of the rotor. Accelerations are subtracted to separate lateral and torsional vibration. The bandwidth of the measurement is in fact very high and sensitivity and accuracy improve with frequency. This technique in some cases may be used, although it requires extremely tight tolerances for relative positioning and systems to acquire signals from rotating sensors.
On the other hand, measurement techniques reducing cost, time, and instrumentation complexity are available. Examples are represented by black/white striped tapes (so called “zebra tapes”) or electromagnetic sensors pointed at phonic wheels, gears/gear rims or bladed rotors. Periods on the rotor (namely the passing stripes, teeth, or blades) are detected and are used to generate a square wave signal, for example a TTL voltage variation. Each wave period is then associated to an angular increment or angular period. As mentioned before, limitations on the accuracy of the stripe size directly translate in a severe limitation to measurement accuracy; the same applies to the accuracy of teeth on rims/gear rims or period of rotor blades or the like. Only in theory are stripes or blocks of a gear rim uniformly spaced.
The measurement error for zebra tapes currently is about one order of magnitude higher than for laser vibrometers and two orders of magnitude higher than the error for optical encoders.